As environmental concerns have led to increasingly strict regulation of engine emissions by governmental agencies, reduction of nitrogen-oxygen compounds (NOx) in exhaust emissions from internal combustion engines has become increasingly important. Current indications are that this trend will continue.
Future emission levels of diesel engines will have to be reduced in order to meet Environmental Protection Agency (EPA) regulated levels. In the past, the emission levels of US diesel engines have been regulated according to the EPA using the Federal Test Procedure (FTP) cycle, with a subset of more restrictive emission standards for California via the California Air Resources Board (CARB). For example, the Tier II emission standards, which are being considered for 2004, are 50% lower than the Tier I standards. Car and light truck emissions are measured over the FTP 75 test and expressed in gm/mi. Proposed Ultra-Low Emissions Vehicle (ULEV) emission levels for light-duty vehicles up to model year 2004 are 0.2 gm/mi NOx and 0.08 gm/mi particulate matter (PM). Beginning with the 2004 model year, all light-duty Low Emission Vehicles (LEVs) and ULEVs in California would have to meet a 0.05 gm/mi NOx standard to be phased in over a three year period. In addition to the NOx standard, a full useful life PM standard of 0.01 gm/mi would also have to be met.
Traditional methods of in-cylinder emission reduction techniques such as exhaust gas recirculation (EGR) and injection rate shaping by themselves will not be able to achieve these low emission levels required by the standard. Aftertreatment technologies will have to be used, and will have to be further developed in order to meet the future low emission requirements of the diesel engine.
NOx adsorber catalysts have the potential for great NOx emission reduction (70-90%) and for extending engine life. However, low temperature operation of adsorbers seems to be a problem. The main reason for this problem seems to be the fact that the reductants, especially D2 fuel, starts to boil at around 180 degrees Celsius. At temperatures below this, the injected fuel has a strong tendency to condense and does not participate in the release and reduction step of NOx adsorbers (catalytic converters). Also, if the droplet size is large, atomization is not good and the fuel does not vaporize easily even at the higher temperature, thereby limiting adsorber performance.
Current devices on their own, such as spraying system nozzles and injectors cannot do anything about the condensation at low temperature. Also, the droplet size measurement shows that they have a sauter mean diameter (SMD) in the range of 30-60 μm. If this droplet size can be reduced further, the vaporization will become faster and easier. This will improve the reductant participation in the catalyst reactions and improve the NOx adsorber capacity and NOx conversion efficiency.
There is therefore a need for a system and method for enhancing internal combustion engine aftertreatment applications that can reduce emission levels for diesel engines. The present invention is directed toward meeting this need.